1. Field of the Invention
The present invention is related to an image forming apparatus such as a copying machine, printer, or facsimile machine using electrophotographic process, specifically to an image forming apparatus having a belt member onto which images are to be formed.
2. Description of the Related Art
Generally, an image forming apparatus is known in which a toner image formed on a belt member is directly or indirectly transferred to a copy sheet.
One of such image forming apparatuses is, for example, a tandem type color image forming apparatus in which a plurality of image forming devices are located along an intermediate transfer belt as the belt member looped over a plurality of rollers. In the image forming apparatus, image forming devices of, for example, black (B), yellow (Y), magenta (M), and cyan (C) are arranged one after another along the intermediate transfer belt in the direction of travel of the belt. Each of the image forming devices is provided with an electric charger, a photoreceptor drum which is an image carrier member, an exposure unit, a developing device, and a cleaning unit, etc. The latent image formed on the photoreceptor drum is developed to a toner image by the developing device and the toner image on the drum is transferred as a primary transferred image to the intermediate transfer belt.
In this way, the toner images of B, Y, M, and C are sequentially transferred to the intermediate transfer belt, and color images are formed on the intermediate transfer belt. The color toner images are transferred to a copy sheet at the secondary image transferring position as a secondary transferred image, then the color image on the copy sheet is permanently fixed to the copy sheet by a fusing unit to be released.
Said intermediate transfer belt is generally unitized as an intermediate image transfer unit as shown in FIG. 4. In FIG. 4, the intermediate image transfer unit has a unit frame 11 to which are attached a driving roller 46, a follower roller 48, and a tension roller 47, the rollers being an example of said plurality of rollers. An intermediate transfer belt 45 is looped over these rollers. The intermediate transfer belt 45 is driven by the driving roller 46 to travel in the direction of an arrow as shown in FIG. 4. A black, yellow, magenta, and cyan image forming devices, for example though not shown in FIG. 4, are arranged between the tension roller 47 and the driving roller 46 one by one above the belt 45 in order from upstream to downstream of the traveling direction of the belt 45, and the transfer of the images on the intermediate transfer belt 45 to a copy sheet (secondary image transfer) is done at the position of the follower roller 46.
In the color image forming apparatus, a concentration sensor for detecting the concentration of the toner image transferred to the intermediate transfer belt 45 (primary transferred image) and a registration sensor for adjusting the timing of the primary transferred image on the intermediate transfer belt to reach the position of second image transferring and the timing of transferring the copy sheet to the position of secondary image transferring, are provided (hereafter, said concentration sensor and registration sensor are together referred to as an image read sensor).
Errors in image formation due to the conditions of the environment in which the image forming apparatus is used and variations in the constituent parts of the image forming apparatus, are eliminated by virtue of the image read sensor.
An optical sensor, for example, is used for the image read sensor. Generally, when using an optical sensor it is necessary to accurately adjust the distance between the sensor and the object to be measured. If the distance from the sensor to the object deviates from the right distance, the sensitivity of the sensor decreases and accurate detection is not possible.
As described above, the intermediate transfer belt 45 is looped over the driving roller 46, follower roller 48, and tension roller 47, the belt being tensioned by means of the tension roller 47 so that it does not slack. On the other hand, the follower roller 48 is located nearer to the tension roller 47 in order to configure the intermediate transfer unit in low profile to realize a compact unit. As a result, the distance between the driving roller 46 and the follower roller 48 (i.e. the position of secondary image transferring) becomes long.
As shown in FIG. 5, an image read sensor 21 is attached to a support member 21a to face toward the intermediate transfer belt 45 at the position between driving roller 46 and follower roller 48 to detect the toner images transferred to the intermediate transfer belt 45.
The intermediate image belt 45 is generally made of rubber material, and the distance between the driving roller 46 and follower roller 48 is very long compared with that between the follower roller 48 and tension roller 47. As a result, there is the case the intermediate transfer belt 45 slacks as shown in FIG. 6 by dashed double dotted line. When the slack of the belt like this occurs, the image on the belt falls out of focus of the image read sensor and the accuracy of detection of image is deteriorated.
The intermediate image transfer unit is mounted detachably on the image forming apparatus to be replaced as needed. When replacing the intermediate image transfer unit, it is necessary to position the intermediate image transfer unit to keep the distance between the intermediate transfer belt 45 and the image read sensor 21 as it was before the replacement. For this, the image read sensor 21 is attached to the bottom face of a frame member 22 fixed to the casing not shown of the image forming apparatus, the frame member 22 having supporting recesses 22a formed on both side walls thereof for supporting the rotation shaft 46a of the driving roller 46 as shown in FIGS. 7(a) and (b). When the intermediate image transfer unit 30 is replaced, the rotation shaft 46a of the new unit is located on the supporting recesses 22a. 
However, even if the positioning of the intermediate image transfer unit 30 is performed as shown in FIG. 7(a), (b) to keep the distance between the belt 45 and the image read sensor 21, if the belt 45 slacks, the surface of the belt 45 can not be brought in focus of the image read sensor 21, resulting in deteriorated accuracy of image reading.
There is disclosed an image forming apparatus, in which the image read sensor is located at the position remotest from the second image transferring position, where the intermediate transfer belt is traveling in the direction perpendicular to the direction of the movement of the secondary image transfer roller for engaging/disengaging with/from the belt and where the toner image transferred onto the belt can be read (see Japanese Laid-Open Patent Application No. 2000-321838, column 0062˜0067, FIG. 1).
It is possible by the method of said disclosure to deal with the vibration of the intermediate transfer belt induced by the engaging/disengaging of the secondary image transfer roller with/from the belt at the second image transferring position, for when the image read sensor is located at the position remotest from the second transferring position and capable of reading the toner image transferred onto the intermediate transfer belt traveling in the direction perpendicular to the direction of engaging/disengaging movement of the secondary image transfer roller with/from the belt, the influence that the vibration in the direction perpendicular to the image plane exerts upon the accuracy of reading image is the smallest. However, the slack of the intermediate transfer belt itself is not taken into consideration, and when the image read sensor faces the belt at the driving roller, there remains a problem that the influence of the driving motion of the driving roller can not be evaded.
Further, in said disclosure no consideration is given as to how the positioning of the intermediate image transfer unit and the image read sensor is done, and moreover there is a problem that it is difficult to locate the image read sensor near the driving roller, for space is limited near the driving roller.
Any way, there are problems that it is not possible to accurately read or detect the image due to the slack of the intermediate transfer belt and that accurate relative positioning of intermediate transfer belt and the image read sensor is difficult.